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absolute zero

The temperature at which significant molecular activity stops. Absolute zero is commonly used by scientists who study what happens to things when they become very cold and is measured as 0 degrees Kelvin -- equal to -459 degrees Fahrenheit or -273 degrees Celsius.

absorption

The process in which radiant energy is retained by a substance. A further process always results from absorption, that is, the irreversible conversion of the absorbed radiation into some other form of energy within and according to the nature of the absorbing medium. The absorbing medium itself may emit radiation, but only after an energy conversion has occurred.

acceleration

Science: in general, any increase in the speed or rate at which some process occur; in technical use acceleration and speed are not synonymous. Mechanics: the vector representing the rate of change in velocity vector over time. It is expressed in meters (or feet) per second per second, and it involves an increase or decrease in speed and a change in direction.

acid rain

Acids form when certain atmospheric gases (primarily carbon dioxide, sulfur dioxide, and nitrogen oxides) come in contact with water in the atmosphere or on the ground and are chemically converted to acidic substances. Oxidants play a major role in several of these acid-forming processes. Carbon dioxide dissolved in rain is converted to a weak acid (carbonic acid). Other gases, primarily oxides of sulfur and nitrogen, are converted to strong acids (sulfuric and nitric acids).

Although rain is naturally slightly acidic because of carbon dioxide, natural emissions of sulfur and nitrogen oxides, and certain organic acids, human activities can make it much more acidic. Occasional pH readings of well below 2.4 (the acidity of vinegar) have been reported in industrialized areas.

The principal natural phenomena that contribute acid-producing gases to the atmosphere are emissions from volcanoes and from biological processes that occur on the land, in wetlands, and in the oceans. The effects of acidic deposits have been detected in glacial ice thousands of years old in remote parts of the globe. Principal human sources are industrial and power-generating plants and transportation vehicles. The gases may be carried hundreds of miles in the atmosphere before they are converted to acids and deposited.

Since the industrial revolution, emissions of sulfur and nitrogen oxides to the atmosphere have increased. Industrial and energy-generating facilities that burn fossil fuels, primarily coal, are the principal sources of increased sulfur oxides. These sources, plus the transportation sector, are the major originators of increased nitrogen oxides.

The problem of acid rain not only has increased with population and industrial growth, it has become more widespread. The use of tall smokestacks to reduce local pollution has contributed to the spread of acid rain by releasing gases into regional atmospheric circulation. The same remote glaciers that provide evidence of natural variability in acidic deposition show, in their more recently formed layers, the increased deposition caused by human activity during the past half century.

ACRIMSAT

The Active Cavity Radiometer Irradiance Monitor (ACRIM) Satellite Mission is a NASA mission to measure Total Solar Irradiance (TSI) over a five-year period. The instrument, third in a series of long-term solar-monitoring tools built for NASA by the Jet Propulsion Laboratory, will continue to extend the database first created by ACRIM I, which was launched in 1980 on the Solar Maximum Mission (SMM) spacecraft. ACRIM II followed on the Upper Atmosphere Research Satellite (UARS) in 1991. See ACRIMSAT Fact Sheet

active system (active sensor)

A remote-sensing system that transmits its own radiation to detect an object or area for observation and receives the reflected or transmitted radiation. Radar is an example of an active system. Compare with passive system.

ASTER is an imaging instrument that will fly on Terra as part of NASA's Earth Observing System (EOS). ASTER will be used to obtain detailed maps of land surface temperature, emissivity, reflectance and elevation. See ASTER Web Site.

A horizontal movement of a mass of fluid, such as ocean or air currents. Can also refer to the horizontal transport of something (e.g., pollution, phytoplankton, ice, or even heat) by such movement.

aerosol

Particles of liquid or solid dispersed as a suspension in gas.

air mass

Large body of air, often hundreds or thousands of miles across, containing air of a similar temperature and humidity. Sometimes the differences between air masses are hardly noticeable, but if colliding air masses have very different temperatures and humidity values, storms can erupt. See front.

air pollution

The existence in the air of substances in concentrations that are determined unacceptable. Contaminants in the air we breathe come mainly from manufacturing industries, electric power plants, automobiles, buses, and trucks.

air pressure

The weight of the atmosphere over a particular point, also called barometric pressure. Average air exerts approximately 14.7 pounds (6.8 kg) of force on every square inch (or 101,325 newtons on every square meter) at sea level.

albedo

The ratio of the outgoing solar radiation reflected by an object to the incoming solar radiation incident upon it.

algae

Simple rootless plants that grow in sunlit waters in relative proportion to the amounts of nutrients available. They are food for fish and small aquatic animals, and a factor in eutrophication.

algal blooms

Sudden spurts of algal growth due to greatly increased amounts of phosphorus entering the aquatic ecosystem from sewage systems and agricultural fertilizers. Excessive growth of the algae causes destruction of many of the higher links of the food web. Algae that die and sink to the bottom at the end of the growing season stimulate massive growth of bacteria the following year, resulting in depletion of oxygen in the deeper water layers. This may result in fish kills and replacement with less valuable species who may be more tolerant of increased phosphorus levels. Deoxygenation also may cause chemical changes in the mud on the bottom, producing increased quantities of chemicals and toxic gases. All these changes further accelerate the eutrophication (aging) of the aquatic ecosystem.

algorithm

A mathematical relation between an observed quantity and a variable used in a step-by-step mathematical process to calculate a quantity.

In the context of remote sensing, algorithms generally specify how to determine higher-level data products from lower-level source data. For example, algorithms prescribe how atmospheric temperature and moisture profiles are determined from a set of radiation observations originally sensed by satellite sounding instruments.

alkaline

Substance capable of neutralizing acid, with a pH greater than 7.0. See pH.

altimeter

An active instrument (see active system) used to measure the altitude of an object above a fixed level. For example, a laser altimeter can measure height from a spacecraft to an ice-sheet. That measurement, coupled with radial orbit knowledge, will enable determination of the topography.

Ames Research Center (ARC)

Located at Moffett Field, California, ARC is active in aeronautical research, life sciences, space science, and technology research. The Center houses the world's largest wind tunnel and the world's most powerful supercomputer system. ARC Web Site

amplitude

The magnitude of the displacement of a wave from a mean value. For a simple harmonic wave, it is the maximum displacement from the mean. For more complex wave motion, amplitude is usually taken as one-half of the mean distance (or difference) between maxima and minima.

anaglyph

images of the same scene in contrasting colors or from different viewing angles that look three-dimensional when they are superimposed

anemometer

Instrument used to measure wind speed, usually measured either from the rotation of wind-driven cups or from wind pressure through a tube pointed into the wind.

anomaly

The deviation of (usually) temperature or precipitation in a given region over a specified period from the normal value for the same region.

The angular distance of an Earth satellite (or planet) from its perigee (or perihelion) as seen from the center of the Earth (sun).

anthropogenic

Made by people or resulting from human activities. Usually used in the context of emissions that are produced as a result of human activities.

anticyclone

A high pressure area where winds blow clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. See cyclone, wind.

aphelion

The point in its orbit when a planet is farthest from the sun. For more information, see Milutin Milankovitch

apogee

On an elliptical orbit path, the point at which a satellite is farthest from the Earth.

The parallel of latitude that is approximately 66.5 degrees north of the equator and that circumscribes the northern frigid zone.

ascending node

The point in an orbit (longitude) at which a satellite crosses the equatorial plane from south to north.

Astronomical Unit (AU)

The distance from the Earth to the sun. On average, the sun is 149,599,000 kilometers from Earth.

atmosphere

The air surrounding the Earth, described as a series of shells or layers of different characteristics. The atmosphere, composed mainly of nitrogen and oxygen with traces of carbon dioxide, water vapor, and other gases, acts as a buffer between Earth and the sun. The layers, troposphere, stratosphere, mesosphere, thermosphere, and the exosphere, vary around the globe and in response to seasonal changes.

Troposphere stems from the Greek word tropos, which means turning or mixing. The troposphere is the lowest layer of the Earth's atmosphere, extending to a height of 8-15 km, depending on latitude. This region, constantly in motion, is the most dense layer of the atmosphere and the region that essentially contains all of Earth's weather. Molecules of nitrogen and oxygen compose the bulk of the troposphere.

The tropopause marks the limit of the troposphere and the beginning of the stratosphere. The temperature above the tropopause increases slowly with height up to about 50 km.

The stratosphere and stratopause stretch above the troposphere to a height of 50 km. It is a region of intense interactions among radiative, dynamical, and chemical processes, in which horizontal mixing of gaseous components proceeds much more rapidly that vertical mixing. The stratosphere is warmer than the upper troposphere, primarily because of a stratospheric ozone layer that absorbs solar ultraviolet energy.

The mesosphere, 50 to 80 km above the Earth, has diminished ozone concentration and radiative cooling becomes relatively more important. The temperature begins to decline again (as it does in the troposphere) with altitude. Temperatures in the upper mesosphere fall to -70 degrees to -140 degrees Celsius, depending upon latitude and season. Millions of meteors burn up daily in the mesosphere as a result of collisions with some of the billions of gas particles contained in that layer. The collisions create enough heat to burn the falling objects long before they reach the ground.

The stratosphere and mesosphere are referred to as the middle atmosphere. The mesopause, at an altitude of about 80 km, separates the mesosphere from the thermosphere--the outermost layer of the Earth's atmosphere.

The thermosphere, from the Greek thermo for heat, begins about 80 km above the Earth. At these high altitudes, the residual atmospheric gases sort into strata according to molecular mass. Thermospheric temperatures increase with altitude due to absorption of highly energetic solar radiation by the small amount of residual oxygen still present. Temperatures can rise to 2,000 degrees C. Radiation causes the scattered air particles in this layer to become charged electrically, enabling radio waves to bounce off and be received beyond the horizon. At the exosphere, beginning at 500 to 1,000 km above the Earth's surface, the atmosphere blends into space. The few particles of gas here can reach 4,500 degrees F (2,500 degrees C) during the day.

Atmospheric Infrared Sounder

Advanced sounding instrument selected to fly on the EOS-PM1 mission (intermediate-sized, sun-synchronous, morning satellite) in the year 2000. It will retrieve vertical temperature and moisture profiles in the troposphere and stratosphere. Designed to achieve temperature retrieval accuracy of 1 degree C with a 1 km vertical resolution, it will fly with two operational microwave sounders. The three instruments will constitute an advanced operational sounding system, relative to the TIROS Operational Vertical Sounder (TOVS) currently flying on NOAA Polar-orbiting satellites. See Earth Observing System, TIROS-N/NOAA Satellites.

atmospheric pressure

The amount of force exerted over a surface area, caused by the weight of air molecules above it. As elevation increases, fewer air molecules are present. Therefore, atmospheric pressure always decreases with increasing height. A column of air, 1 square inch in cross section, measured from sea level to the top of the atmosphere would weigh approximately 14.7 lb/in2. The standard value for atmospheric pressure at sea level is:

29.92 inches or 760 mm of mercury

1013.25 millibars (mb) or 101,325 pascals (pa).

Atmospheric Radiation Measurements Program (ARM)

U.S. Department of energy program for the continual, ground-based measurements of atmospheric and meteorological parameters over approximately a ten-year period. The program will study radiative forcing and feedbacks, particularly the role of clouds. The general program goal is to improve the performance of climate models, particularly general circulation models of the atmosphere.

atmospheric response variables

Variables that reflect the response of the atmosphere to external forcing (e.g., temperature, pressure, circulation, and precipitation).

atmospheric windows

The range of wavelengths at which water vapor, carbon dioxide, or other atmospheric gases only slightly absorb radiation. Atmospheric windows allow the Earth's radiation to escape into space unless clouds absorb the radiation. See greenhouse effect.

atoll

A coral island consisting of a ring of coral surrounding a central lagoon. Atolls are common in the Indian and Pacific Oceans.

attenuation

The decrease in the magnitude of current, voltage, or power of a signal in transmission between points. Attenuation may be expressed in decibels, and can be caused by interference?s such as rain, clouds, or radio frequency signals.

azimuth

The direction, in degrees referenced to true north, that an antenna must be pointed to receive a satellite signal (compass direction). The angular distance is measured in a clockwise direction.